"empty" bucket name should be treated as NULL

Change-Id: Ib39487055f2cffea58bdc03f99c726e84c827a19
Reviewed-on: http://review.couchbase.org/12559
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>

src/instance.c

@@ -68,7 +68,7 @@ libcouchbase_t libcouchbase_create(const char *host,
         host = "localhost";
     }
 
-    if (bucket == NULL) {
+    if (bucket == NULL || strlen(bucket) == 0) {
         bucket = "default";
     }